In modern cellular communication systems, so called MIMO technology, multiple-input multiple-output, is often used in order to enhance the performance of the system.
MIMO systems are known to outperform non-MIMO systems if the wireless medium supports a so called “rich” communications channel, i.e. one with a significant amount of multipath propagation between the transmitting and the receiving party.
A high degree of multipath propagation typically leads to a so called “high-rank” MIMO channel, which can be used for an increase in the data rate by transmission of multiple simultaneous data streams or for increased error protection by using diversity techniques over the MIMO channel.
However, if there is a low degree of multipath propagation, the MIMO channel might be “rank deficient”, and therefore not able to support the desired kinds of MIMO techniques. This is especially the case in pure line-of-sight, LOS, conditions, where there is no multipath propagation, or the reflected paths are too weak compared to the direct path, so that the MIMO channel becomes practically rank deficient.
In addition to requiring high rank channels, MIMO techniques also typically require high signal-to-noise ratio, SNR, in order to provide good performance.
However, a rich multipath environment, which is associated with high rank channels, typically also lead to strong signal attenuation due to multiple reflections, etc. and, thus, too low SNR.